Hydraulic system for work machine and work machine

ABSTRACT

A hydraulic system for a work machine includes a tank to store an operation fluid, a hydraulic device to be operated by the operation fluid, a control valve to control the hydraulic device, a first fluid tube connecting the hydraulic device and the control valve, the first fluid tube being to supply the operation fluid from the control valve to the hydraulic device, a second fluid tube branching from the first fluid tube and connected to the tank, a switch valve provided to the second fluid tube, the switch valve being to control a flow rate of the operation fluid, and an oil cooler provided to the second fluid tube between the switch valve and the tank.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S.application Ser. No. 15/634,219, filed Jun. 27, 2017, which is acontinuation of International Application No. PCT/JP2015/086509, filedDec. 28, 2015, and claims priority to Japanese Patent Application No.2015/078504, filed Apr. 7, 2015. The contents of these applications areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a hydraulic system for a work machineand the work machine having the hydraulic system.

Description of Related Art

Japanese Unexamined Patent Application Publication No. 2013-36274discloses a work machine such as a skid steer loader and a compact trackloader to which an auxiliary attachment is attached.

The work machine disclosed in Japanese Unexamined Patent ApplicationPublication No. 2013-36274 includes a control valve configured tocontrol a hydraulic actuator of the auxiliary attachment. A joint isconnected to the control valve through a fluid tube. A hydraulic hose ofthe hydraulic actuator is capable of being connected to the joint. Inthis manner, the control valve is operated, and thereby the hydraulicactuator is operated.

BRIEF SUMMARY OF THE INVENTION

A hydraulic system for a work machine includes a tank to store anoperation fluid, a hydraulic device to be operated by the operationfluid, a control valve to control the hydraulic device, a first fluidtube connecting the hydraulic device and the control valve, the firstfluid tube being to supply the operation fluid from the control valve tothe hydraulic device, a second fluid tube branching from the first fluidtube and connected to the tank, a switch valve provided to the secondfluid tube, the switch valve being to control a flow rate of theoperation fluid, and an oil cooler provided to the second fluid tubebetween the switch valve and the tank.

A hydraulic system for a work machine includes a hydraulic device to beoperated by an operation fluid, a control valve to control the hydraulicdevice, a first fluid tube connecting the hydraulic device and thecontrol valve, the first fluid tube being to supply the operation fluidfrom the control valve to the hydraulic device, a second fluid tubeconnected to the first fluid tube, the second fluid tube being capableof draining the operation fluid of the first fluid tube, a firstoperation valve provided to the second fluid tube and configured tochange an aperture of the first operation valve, including apressure-receiving portion to receive a pressure of a pilot fluid thatis a part of the operation fluid used for control, a seventh fluid tubeconnected to the pressure-receiving portion of the first operationvalve, a second operation valve provided to the seventh fluid tube andconfigured to change an aperture of the second operation valve, thesecond operation valve being to change the pressure of the pilot fluidto be applied to the pressure-receiving portion of the first operationvalve in accordance with the aperture.

A hydraulic system for a work machine includes a working hydraulic pumpto output an operation fluid, an extending hydraulic pump to output theoperation fluid, a hydraulic device to be operated by the operationfluid, a control valve to control the hydraulic device, a first fluidtube connecting the hydraulic device and the control valve, the firstfluid tube being to supply the operation fluid from the control valve tothe hydraulic device, a second fluid tube connected to the first fluidtube, the second fluid tube being capable of draining the operationfluid of the first fluid tube, an eighth fluid tube branching the firstfluid tube and connected to the extending hydraulic pump, a firstoperation valve including a pressure-receiving portion to receive apressure of a pilot fluid that is a part of the operation fluid used forcontrol, the first operation valve being provided to the second fluidtube and configured to change an aperture of the first operation valvein accordance with the pressure of the pilot fluid applied to thepressure-receiving portion, a third operation valve including apressure-receiving portion to receive the pressure of the pilot fluid,the third operation valve being provided to the eighth fluid tube andconfigured to change an aperture of the third operation valve, a seventhfluid tube connected to the pressure-receiving portion of the firstoperation valve, and a ninth fluid tube connected to thepressure-receiving portion of the third operation valve and to theseventh fluid tube.

A work machine includes any one of the hydraulic systems mentionedabove, a machine body, a working device disposed on the machine body, aconnection member disposed on the working device and on an intermediateportion of the first fluid tube included in the hydraulic system,wherein a branching portion where the second fluid tube of the hydraulicsystem branches from the first fluid tube is disposed between theconnection member and the control valve.

A work machine includes the hydraulic system mentioned above, a machinebody, a working device disposed on the machine body, a connection memberdisposed on the working device and on an intermediate portion of thefirst fluid tube included in the hydraulic system, wherein a branchingportion where the eighth fluid tube of the hydraulic system branchesfrom the first fluid tube is disposed between the connection member andthe control valve.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic view illustrating a traveling hydraulic systemaccording to a first embodiment of the present invention;

FIG. 2 is a schematic view illustrating a working hydraulic systemaccording to the first embodiment;

FIG. 3 is a schematic view illustrating a working hydraulic systemaccording to a second embodiment of the present invention;

FIG. 4A is a view illustrating a first modified example of the secondembodiment;

FIG. 4B is a view illustrating a second modified example of the secondembodiment;

FIG. 5 is a view illustrating a third modified example of the secondembodiment;

FIG. 6 is a view a schematic view illustrating a working hydraulicsystem according to a third embodiment of the present invention;

FIG. 7A is a schematic view illustrating a first working hydraulicsystem according to a fourth embodiment of the present invention;

FIG. 7B is a view illustrating a second working hydraulic systemaccording to the fourth embodiment;

FIG. 8A is a schematic view illustrating a working hydraulic systemaccording to a fifth embodiment of the present invention;

FIG. 8B is a view illustrating a modified example of the fifthembodiment;

FIG. 9 is a schematic view illustrating a working hydraulic systemaccording to a sixth embodiment of the present invention;

FIG. 10 is a view illustrating a modified example of the workinghydraulic system according to the sixth embodiment;

FIG. 11 is a side view illustrating a track loader that is an example ofa work machine according to the embodiments of the present invention;and

FIG. 12 is a side view illustrating a part of the track loader liftingup a cabin according to the embodiments.

DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanyingdrawings, wherein like reference numerals designate corresponding oridentical elements throughout the various drawings. The drawings are tobe viewed in an orientation in which the reference numerals are viewedcorrectly.

Embodiments of the present invention, a hydraulic system for a workmachine and the work machine having the hydraulic system, will beexplained below referring to the drawings.

First Embodiment

A general configuration of a work machine according to a firstembodiment of the present invention will be explained first. As shown inFIG. 11 and FIG. 12, the work machine 1 includes a machine body 2, acabin 3, a work device 4, and a travel device 5.

FIG. 11 and FIG. 12 illustrate a compact track loader as an example ofthe work machine 1. However, the work machine 1 according to theembodiment is not limited to a compact track loader, and may be atractor, a skid steer loader, a backhoe, for example.

Hereinafter, in explanations of all the embodiments of the presentinvention, a forward direction (a direction toward a left side in FIG.11) corresponds to a front side of an operator seated on an operatorseat of the work machine 1, a backward direction (a direction toward aright side in FIG. 11) corresponds to a back side of the operator, aleftward direction (a direction from the back toward a front side ofFIG. 11) corresponds to a left side of the operator, and a rightwarddirection (a direction from the front toward a back side of FIG. 11)corresponds to a right side of the operator.

The cabin 3 is mounted on the machine body 2. An operator seat 8 isdisposed inside the cabin 3. The work device 4 is attached to themachine body 2. The travel device is arranged outside the machine body2. A prime mover is mounted on a rear portion inside the machine body 2.

The work device 4 includes a boom 10, a work tool 11, a lift link 12, acontrol link 13, a boom cylinder 14, and a bucket cylinder 15.

The boom 10 is arranged to the right of the cabin 3, and is capable ofbeing swung upward and downward. Another boom 10 is arranged to the leftof the cabin 3, and is capable of being swung upward and downward. Thework tool 11 is constituted of a bucket, for example. The bucket 11 isarranged on a tip end portion (a front end portion) of the boom 10, andis capable of being swung upward and downward.

The lift link 12 and the control link 13 support a base portion (a rearportion) of the boom 10 such that the boom 10 is capable of being swungupward and downward. The boom cylinder 14 is stretched and shortened tomove the boom 10 upward and downward. The bucket cylinder 15 isstretched and shortened to swing the bucket 11.

A front portion of the boom 10 arranged to the left is connected by adeformed connection pipe to a front portion of the boom 10 arranged tothe right. A base portion (a rear portion) of the boom 10 arranged tothe left is connected by a cylindrical connection pipe to a base portion(a rear portion) of the boom 10 arranged to the right.

The lift links 12, the control links 13, and the boom cylinders 14 arearranged to the left of the machine body 2 and to the right of themachine body 2, corresponding to the boom 10 disposed on the left andthe boom 10 disposed on the right.

The lift links 12 are disposed on the rear portions of the base portionsof the booms 10, and extend in a vertical direction. The upper portions(one end sides) of the lift links 12 are pivotally supported by pivotalsupports shafts 16 (first pivotal support shafts), being closer to therear portions of the base portions of the booms 10, and are capable ofturning about the lateral axis.

In addition, the lower portions (the other end sides) of the lift links12 are pivotally supported by pivotal supports shafts 17 (second pivotalsupport shafts), being closer to the rear portion of the machine body 2,and are capable of turning about the lateral axis. The second pivotalsupport shafts 17 are arranged below the first pivotal support shafts16.

The upper portions of the boom cylinders 14 are pivotally supported bythe pivotal support shafts 18 (third pivotal support shafts), and arecapable of turning about the lateral axis. The third pivotal supportshafts 18 are disposed on the base portions of the booms 10,specifically on the front portions of the base portions.

The lower portions of the boom cylinder 14 are pivotally supported bythe pivotal support shafts 19 (fourth pivotal support shafts), and arecapable of turning about the lateral axis. The fourth pivotal supportshafts 19 are disposed below the third pivotal support shafts 18, beingcloser to the lower portion of the rear portion of the machine body 2.

The control links 13 are arranged in front of the lift links 12. Oneends of the control links 13 are pivotally supported by the pivotalsupports shafts 20 (fifth pivotal supports shafts), and are capable ofturning about the lateral axis. The fifth pivotal support shafts 20 aredisposed on the machine body 2, specifically on corresponding positionsin front of the lift links 12. Th other ends of the control links 12 arepivotally supported by the pivotal supports shafts 21 (sixth pivotalsupports shafts), and are capable of turning about the lateral axis. Thesixth pivotal support shafts 21 are disposed on the booms 10 in front ofthe second pivotal support shafts 17 and above the second pivotalsupport shafts 17.

When the boom cylinders 14 are stretched and shortened, the booms 10swing upward and downward about the first pivotal support shafts 6 withthe base portions of the booms 10 supported by the lift links 12 and thecontrol links 13, and thus the tip end portions of the booms 10 moveupward and downward.

The control links 13 swing upward and downward about the fifth pivotalsupport shafts 20 in accordance with the upward swinging and thedownward swinging of the booms 10. The lift links 12 swing forward andbackward about the second pivotal support shafts 17 in accordance withthe upward swinging and the downward swinging of the control links 13.

The front portions of the booms 10 are capable of attaching other worktools instead of the bucket 11. The following attachments (auxiliaryattachments) are exemplified as the other work tools; for example, ahydraulic crusher, a hydraulic breaker, an angle broom, an earth auger,a pallet fork, a sweeper, a mower, a snow blower and the like.

A connection member 50 is disposed on the front portion of the boom 10disposed on the left. The connection member 50 is a device forconnecting a hydraulic device of an auxiliary attachment disposed on afirst tube member pipe such as a pipe disposed on the boom 10.

Specifically, the first tube member is capable of being connected to oneend of the connection member 50, and a second tube member is capable ofbeing connected to the other end of the connection member 50, the secondtube member being connected to the hydraulic device of the auxiliaryattachment. In this manner, an operation fluid flowing in the first tubemember is supplied to the hydraulic device through the second tubemember.

The bucket cylinders 15 are arranged on portions close to the frontportions of the booms 10. The bucket cylinders 15 are stretched andshortened to swing the bucket 11.

Each of the travel device 5 disposed on the left and the travel device 5disposed on the right employs a travel device of a crawler type(including a semi-crawler type) in the embodiment. Each of the traveldevices 5 may employ a travel device of a wheel type having the frontwheels and the rear wheels.

The hydraulic system for the work machine according to the embodimentwill be explained below.

As sown in FIG. 1 and FIG. 2, the hydraulic system is roughly separatedinto a hydraulic system for travel (a travel hydraulic system) 30A and ahydraulic system for work (a work hydraulic system) 30B.

The travel hydraulic system 30A will be explained below.

As shown in FIG. 1, the travel hydraulic system 30A is a system fordriving the travel device 5. The travel device 5 mainly includes a lefttravel motor device 31L (a first travel motor device) and a right travelmotor device 31R (a second travel motor device). The travel hydraulicsystem 30A includes a prime mover 32, a direction switch valve 33, atravel hydraulic pump (a first hydraulic pump) P1, a first travel motor31L, a second travel motor 31R, and a hydraulic drive device 34.

The prime mover 32 is constituted of an electric motor, an engine, orthe like. In the embodiment, the prime mover 32 is the engine. The firsthydraulic pump P1 is a pump configured to be driven by a driving forceof the prime mover 32. The first hydraulic pump P1 is constituted of aconstant displacement gear pump.

The first hydraulic pump P1 is configured to output the operation fluidstored in the tank 22. In particular, the first hydraulic pump P1outputs the operation fluid mainly used for the control. For convenienceof the explanation, the tank 22 for storing the operation fluid may bereferred to as an operation fluid tank. In addition, of the operationfluid outputted from the first hydraulic pump P1, the operation fluidused for the control is referred to as a pilot fluid, and a pressure ofthe pilot fluid is referred to as a pilot pressure.

An output fluid tube (an output fluid path) 40 is disposed on an outputside of the first hydraulic pump P1, the output fluid tube 40 beingconfigured to supply the operation fluid (the pilot fluid). The outputfluid tube (a first fluid tube) 40 is provided with a filter 35, thedirection switch valve 33, the first travel motor device 31L, and thesecond travel motor device 31R.

A first charge fluid tube 41 is arranged between the filter 35 and thedirection switch valve 33, the first charge fluid tube 41 being branchedfrom the output fluid tube 40. The first charge fluid tube 41 reachesthe hydraulic drive device 34.

The direction switch valve 33 is an electromagnetic valve configured tochange revolutions of the first travel motor device 31L and the secondtravel motor device 31R. The direction switch valve 33 is constituted ofa two-position switch valve being switched to a first position 33 a andto a second position 33 b by magnetization. The direction switch valve33 is switched by an operation member such as a switch not shown in thedrawings,

The first travel motor device 31L is a motor configured to transmit amotive power to a drive shaft of the travel device 5, the travel device5 being arranged to the left of the machine body 2. The second travelmotor device 31R is a motor configured to transmit a motive power to adrive shaft of the travel device 5, the travel device 5 being arrangedto the right of the machine body 2.

The first travel motor device 31L includes an HST motor (a travel motor)36, a swash-plate switch cylinder 37, and a travel control valve (ahydraulic switch valve) 38. The HST motor 36 is a variable displacementaxial motor having a swash plate, and is a motor capable of changing avehicle speed (revolution) to a first speed and to a second speed. Inother words, the HST motor 36 is a motor capable of changing a thrustpower of the work machine 1.

A sixth fluid tube (a sixth fluid path) 80 is disposed on the HST motor36. The sixth fluid tube 80 is a fluid tube for supplying the operationfluid leaking from the HST motor 36 for example, and extends toward theoperation fluid tank 22.

One end of the sixth fluid tube 80 is connected to a drain port of theHST motor 36. The other end of the sixth fluid tube 80 is connected toan oil cooler (a fluid cooler) 81 configured to cool the operationfluid. In this manner, the operation fluid heated up by the revolutionof the HST motor 36 reaches the oil cooler 81, passing through the sixthfluid tube 80, and thus the oil cooler 81 can cool the operation fluid.

Then, the operation fluid cooled by the oil cooler 81 returns to theoperation fluid tank 22.

The swash-plate switch cylinder 37 is a cylinder configured to bestretched and shortened to change an angle of the swash plate of the HSTmotor 36. The travel control valve 38 is a valve for stretching andshortening the swash-plate switch cylinder 37 to one side and to theother side, that is, the travel control valve 38 is constituted of atwo-position switch valve configured to be switched to a first position38 a and to a second position 38 b. The travel control valve 38 isswitched by the direction switch valve 33 that is connected to thetravel control valve 38 and arranged on an upper stream of the travelcontrol valve 38.

As described above, when the switch (the operation member) is operatedto switch the direction switch valve 33 to the first position 33 a, thefirst travel motor 31L releases the pilot fluid in a section between thedirection switch valve 33 and the travel control valve 38, and thus thetravel control valve 38 is switched to the first position 38 a. As theresult, the swash-plate switch cylinder 37 is shortened, and thus theHST motor 36 is set to the first speed.

In addition, when the switch (the operation member) is operated toswitch the direction switch valve 33 to the second position 33 b, thepilot fluid is supplied to the travel control valve 38 through thedirection switch valve 33, and thus the travel control valve 38 isswitched to the second position 38 b. As the result, the swash-plateswitch cylinder 37 is stretched, and thus the HST motor 36 is set to thesecond speed.

Meanwhile, the second travel motor device 31R is operated in the mannersimilar to the manner of the first travel motor device 311. Theconfigurations and movements of the second travel motor device 31R issimilar to the configurations and movements of the first travel motordevice 31L. Thus, the explanation of the second travel motor device 31Rwill be omitted.

The hydraulic device 34 is a device configured to drive the first travelmotor device 31L and the second travel motor device 31R. The hydraulicdevice 34 includes a drive circuit (a left drive circuit) 34L and adrive circuit (a right drive circuit) 34R. The drive circuit 34L isconfigured to drive the first travel motor device 31L. The drive circuit34R is configured to drive the second travel motor device 31R.

The drive circuit 34L includes an HST pump (a travel pump) 53, aspeed-changing fluid tube (a speed-changing fluid path) 100 h, aspeed-changing fluid tube (a speed-changing fluid path) 100 i, and asecond charging fluid tube (a second charging fluid path) 100 j. Thedrive circuit 34R includes another HST pump (a travel pump) 53, thespeed-changing fluid tube 100 h, the speed-changing fluid tube 100 i,and the second charging fluid tube 100 j. The speed-changing fluid tubes100 h and 100 i are fluid tubes (fluid paths) connecting the HST pumps53 to the HST motor 36.

The second charging fluid tube 100 j is a fluid tube (a fluid path)connected to the speed-changing fluid tubes 100 h and 100 i, and isconfigured to charge the operation fluid from the first hydraulic pumpP1 to the speed-changing fluid tubes 100 h and 100 i.

Each of the HST pumps 53 is the variable displacement axial pump havinga swash plate. The variable displacement axial pump is configured to hedriven by a motive power of the prime mover 32. Each of the HST pumps 53includes a forward-movement pressure-receiving portion 53 a (apressure-receiving portion 53 a) and a backward-movementpressure-receiving portion 53 b (a pressure-receiving portion 53 b). Thepilot pressure is applied to the forward-movement pressure-receivingportion 53 a and the backward-movement pressure-receiving portion 53 b.An angle of the swash plate is changed by the pilot pressure applied tothe pressure-receiving portion 53 a and the pressure-receiving portion53 b.

When the angle of the swash plate is changed, the changing changes theoutputs (output amounts of the operation fluid) of the HST pumps 53 andchanges the directions of the outputs of the operation fluid.

A travel lever 54 changes the outputs of the HST pumps 53 and thedirections of the outputs of the operation fluid. The travel lever 54 isarranged around the operator seat 8. The travel lever 54 is swingablysupported to be capable of being tilted from an neutral position towarda forward, a backward, a leftward, and. a rightward directions and thediagonal directions each between the forward, the backward, theleftward, and the rightward directions.

When the travel lever 54 is operated to be tilted, the pilot valves 55arranged under the travel lever 54 are operated the travel lever 54.

When the operation lever 54 is tilted forward (in one direction) in theforward and backward directions (the first direction), that is, theoperation lever 54 is operated in a forward operation, aforward-movement operation valve 55. A of the operation valves 55 isoperated to output the pilot pressure. The pilot pressure is applied tothe forward-movement pressure-receiving portion 53 a of the left drivecircuit 34L and to the forward-movement pressure-receiving portion 53 aof the right drive circuit 34R.

In this manner, an output shaft of the HST motor 36 revolves normally(revolves forward) at a speed proportional to a tilting amount (atilting extent) of the travel lever 54, and thereby the work machine 1travels forward and straight.

When the operation lever 54 is tilted backward (in the other direction)in the forward and backward directions (the first direction), that is,the operation lever 54 is operated in a backward operation, abackward-movement operation valve 55A of the operation valves 55 isoperated to output the pilot pressure. The pilot pressure is applied tothe backward-movement pressure-receiving portion 53 b of the left drivecircuit 34L and to the backward-movement pressure-receiving portion 53 bof the right drive circuit 34R.

In this manner, the output shaft of the HST motor 36 revolves reversely(revolves backward) at a speed proportional to a tilting amount (atilting extent) of the travel lever 54, and thereby the work machine 1travels backward and straight.

When the operation lever 54 is tilted rightward (in one direction) inthe lateral direction (the second direction), that is, the operationlever 54 is operated in a rightward operation, a rightward-movementoperation valve 55C of the operation valves 55 is operated to output thepilot pressure. The pilot pressure is applied to the forward-movementpressure-receiving portion 53 a of the left drive circuit 34L and to thebackward-movement pressure-receiving portion 53 b of the right drivecircuit 34R.

In this manner, the output shaft of the HST motor 36 arranged to theleft revolves normally (revolves forward), the output shaft of the HSTmotor 36 arranged to the right revolves reversely (revolves backward),and thereby the work machine 1 turns rightward.

When the operation lever 54 is tilted leftward (in the other direction)in the lateral direction (the second direction), that is, the operationlever 54 is operated in a leftward operation, a leftward-movementoperation valve 55D of the operation valves 55 is operated to output thepilot pressure. The pilot pressure is applied to the forward-movementpressure-receiving portion 53 a of the right drive circuit 34R and tothe backward-movement pressure-receiving portion 53 b of the left drivecircuit 34L.

In this manner, the output shaft of the HST motor 36 arranged to theright revolves normally (revolves forward), the output shaft of the HSTmotor 36 arranged to the left revolves reversely (revolves backward),and thereby the work machine 1 turns leftward.

In addition, when the operation lever 54 is tilted in the diagonaldirection, turning directions and turning speeds of the output shafts ofthe travel motor 36 arranged to the left side and the travel motor 36arranged to the right side are determined by a differential pressurebetween the pilot pressure applied to the pressure-receiving portion 53a and the pilot pressure applied to the pressure-receiving portion 53 b,and thus the work machine 1 turns rightward or leftward travelingforward or backward.

The work hydraulic system 30B will be explained below.

As shown in FIG. 2, the work hydraulic system 30B is a system configuredto operate the booms 10, the bucket 11, an auxiliary attachment, and thelike. The work hydraulic system 30B includes a plurality of controlvalves 56 and a work operation hydraulic pump (a second hydraulic pump)P2.

The second hydraulic pump P2 is a pump configured to be driven by amotive power of the prime mover 32 and arranged on a position differentfrom the position of the first hydraulic pump P1. The second hydraulicpump P2 is constituted of a various displacement axial pump having aswash plate.

The second hydraulic pump P2 is configured to output the operation fluidstored in the operation fluid tank 22. In particular, the secondhydraulic pump P2 outputs the operation fluid mainly used for operatingthe hydraulic actuators.

A fluid tube (a fluid path) 39 is disposed on an output side of thesecond hydraulic pump P2. The plurality of control valves 56 areconnected to the fluid tube 39. As shown in FIG. 2, the plurality ofcontrol valves 56 include a first control valve 56A, a second controlvalve 56B, and a third control valve 56C.

The first control valve 56A is a valve configured to control thehydraulic cylinder (the boom cylinder) 14 for controlling the boom. Thesecond control valve 56B is a valve configured to control the hydrauliccylinder (the bucket cylinder) 15 for controlling the bucket.

Operations of the boom 10 and the bucket 11 will be explained first.

Each of the boom control valve 56A and the bucket control valve 56B isconstituted of a three-position switch valve having a direct-actingspool that is configured to be driven by the pilot pressure. Each of theboom control valve 56A and the bucket control valve 56B is switched bythe pilot pressure to a neutral position, to a first position differentfrom the neural position, and to a second position different from theneutral position and the first position.

The boom cylinder 14 is connected to the boom control valve 56A by afluid tube. The bucket cylinder 15 is connected to the second controlvalve 56B by a fluid tube.

The boom 10 and the bucket 11 are operated by an operation lever 58arranged around the operator scat 8. The operation lever 58 is swingablysupported to be capable of being tilted from an neutral position towarda forward and a backward directions, a leftward and a rightwarddirections, and the diagonal directions. When the operation lever 58 istilted, and thereby the pilot valves arranged under the operation lever58 are operated.

When the operation lever 58 is tilted forward (a forward operation isperformed), a downward-movement pilot valve 59A is operated to outputthe pilot pressure. The pilot pressure is applied to thepressure-receiving portion of the boom control valve 56A. In thismanner, the boom 10 moves downward.

When the operation lever 58 is tilted backward (a backward operation isperformed), an upward-movement pilot valve 59B is operated to output thepilot pressure. The pilot pressure is applied to the pressure-receivingportion of the boom control valve 56A. In this manner, the boom 10 movesupward.

When the operation lever 58 is tilted rightward (a rightward operationis performed), an bucket-dumping pilot valve 59C is operated, and thusthe pilot fluid is applied to the pressure-receiving portion of thebucket control valve 56B. As the result, the bucket control valve 56B isactuated to stretch the bucket cylinder 15, and thus the bucket 11performs a dumping movement at a speed proportional to the tiltingamount (the tilting extent) of the operation lever 58.

When the operation lever 58 is tilted leftward (a leftward operation isperformed), an bucket-shoveling pilot valve 59D is operated, and thusthe pilot fluid is applied to the pressure-receiving portion of thebucket control valve 56B. As the result, the bucket control valve 56B isactuated to shorten the bucket cylinder 15, and thus the bucket 11performs a shoveling movement at a speed proportional to the tiltingamount (the tilting extent) of the operation lever 58.

A fifth fluid tube (a fifth fluid path) 82 is connected to the drainsides (the drain ports) of the boom control valve 56A, the bucketcontrol valve 56B, and the auxiliary control valve 56C, the drain side(the drain ports) being configured to drain the operation fluid. Thefifth fluid tube 82 is a fluid tube (a fluid path) for supplying theoperation fluid to the operation fluid tank 22, the operation fluidbeing drained from the control valves (the boom control valve 56A, thebucker control valve 56B, the auxiliary control valve 56C).

In this manner, the operation fluid returning from the boom cylinder 14to the boom control valve 56A (a returning operation fluid), theoperation fluid returning from the bucket cylinder 15 to the bucketcontrol valve 56B (a returning operation fluid), and the operation fluid(the drain) returning from the hydraulic device to the auxiliary controlvalve 56C (a returning operation fluid) are supplied to the fifth fluidtube 82.

The fifth fluid tube 82 is connected indirectly to the second fluid tube(the third drain fluid tube 84 c) through the oil cooler 81 as describedlater. The fifth fluid tube 82 is provided with a first check valve 77.The first check valve 77 allows the operation fluid having apredetermined pressure or more to flow from a side of the fifth fluidtube 82 toward the second fluid tube 84, and block the operation fluidflowing from the side of the second fluid tube 84 toward the controlvalves (the boom control valve 56A, the bucket control valve 56B, andthe auxiliary control valve 56C).

A setup pressure of the first check valve 77 is set to be from 2 kgf/cm²to 5 kgf/cm². The control valve unit including the plurality of controlvalves is provided with the fifth fluid tube 82. However, the firstcheck valve 77 may be arranged in the vicinity of a tank port (a portfor draining the operation fluid) in the control valve unit, and may bearranged on an intermediate portion of a tube member connected to thetank port from the outside of the control valve unit.

The auxiliary control valve 56C and the fluid tubes arranged around theauxiliary control valve 56C will be explained in detail below.

The auxiliary control valve 56C is constituted of a three-positionswitch valve having a direct-acting spool that is configured to bedriven by the pilot pressure. The auxiliary control valve 56C isswitched by the pilot pressure to a first position 91 a, a secondposition 91 b, and a third position (a neutral position) 91 c.

That is, the auxiliary control valve 56C is a valve configured to beswitched to the first position 91 a, to the second position 91 b, and tothe third position 91 c and thereby to control a flow rate and apressure of the operation fluid flowing toward the hydraulic device ofthe auxiliary attachment. That is, the auxiliary control valve 56C is avalve configured to control the hydraulic device.

The hydraulic device is a device for driving the auxiliary attachmentssuch as the hydraulic crusher, the hydraulic breaker, the angle broom,the earth auger, the pallet fork, the sweeper, the mower, the snowblower, and is operated by the operation fluid. For example, thehydraulic device is constituted of a hydraulic cylinder, a hydraulicmotor, or the like.

A first fluid tube (a first fluid path) 83 is connected to the auxiliarycontrol valve 56C. One end of the first fluid tube 83 is connected to asupply-drain port of the auxiliary control valve 56C. An intermediateportion of the first fluid tube 83 is connected to the connection member50. The other end of the first fluid tube 83 is connected to thehydraulic device of the auxiliary hydraulic actuator. The first fluidtube 83 is constituted of the first tube member and the second tubemember described above.

In particular, the first fluid tube 83 includes a first supply-discharge(supply-drain) fluid tube 83 a that connects a first supply-discharge(supply-drain) port of the auxiliary control valve 56C to a first portof the connection member 50. In addition, the first fluid tube 83includes a second supply-discharge (supply-drain) fluid tube 83 b thatconnects a second supply-discharge port of the third control valve 56Cto a second port of the connection member 50.

That is, the operation of the auxiliary control valve 56C allows tosupply the operation fluid from the auxiliary control valve 56C towardthe first supply-drain fluid tube 83 a and to supply the operation fluidfrom the auxiliary control valve 56C toward the second supply-dischargefluid tube 83 b.

The auxiliary control valve 56C is operated by two electromagneticvalves, a first proportional valve 60A and a second proportional valve60B. The auxiliary control valve 56C is connected to the electromagneticvalves (the first electromagnetic valve 60A and the secondelectromagnetic valve 60B) by a third fluid tube (a third fluid path)86.

Meanwhile, the first hydraulic pump P1 supplies the pilot fluid to thefirst electromagnetic valve 60A and the second electromagnetic valve60B, the pilot fluid being the operation fluid used for control that isa part of the operation fluid.

The third fluid tube 86 is a fluid tube for supplying the pilot fluid tothe auxiliary control valve 56C through the electromagnetic valves 60(the first electromagnetic valve 60A and the second electromagneticvalve 60B). The third fluid tube 86 includes a first control fluid tube(a first control fluid path) 86 a and a second control fluid tube (asecond control fluid path) 86 b. The first control fluid tube 86 aconnects the first electromagnetic valve 60A. to the pressure-receivingportion 90 a of the auxiliary control valve 56C. The second controlfluid tube 86 b connects the second electromagnetic valve 60B to thepressure-receiving portion 90 b of the auxiliary control valve 56C.

Each of the first electromagnetic valve 60A and the secondelectromagnetic valve 60B is a valve configured to he magnetized tochange the aperture of each of the first electromagnetic valve 60A andthe second electromagnetic valve 60B. The first electromagnetic valve60A and the second electromagnetic valve 60B are magnetized by a controldevice (a controller) 98 constituted of a CPU or the like. Inparticular, an operation member 99 is connected to the controller 98.

An operation amount (an operation extent) of the operation member 99(for example, a sliding amount, a swinging amount, or the like) isinputted to the control device 98. The operation member 99 isconstituted of a seesaw switch configured to be swung, a slide switchconfigured to be slid, or a push switch configured to be pushed.

The controller 98 applies an electric current to the solenoid of thefirst electromagnetic valve 60A or to the solenoid of the secondelectromagnetic valve 60B, the electric current corresponding to theoperation amount of the operation member 99. That is, the firstelectromagnetic valve 60A and the second electromagnetic valve 60Bchange the apertures corresponding to the operation amount of theoperation member 99.

For example, the aperture of the first electromagnetic valve 60A isadjusted by swinging or sliding the operation member 99 in onedirection. In that case, when the pressure of the pilot fluid (the pilotpressure) first control fluid tube 86 a is equal to a predeterminedvalue (the setup value) or more, the spool of the auxiliary controlvalve 56C moves to switch the auxiliary control valve 56C to the firstposition 91 a.

In addition, the aperture of the second electromagnetic valve 60B isadjusted by swinging or sliding the operation member 99 in the otherdirection. In that case, when the pressure of the pilot fluid (the pilotpressure) in the second control fluid tube 86 b is equal to apredetermined value (the setup value) or more, the spool of theauxiliary control valve 56C moves to switch the auxiliary control valve56C to the second position 91 b.

When the auxiliary control valve 56C is at the first position 91 a, theoperation fluid flows in the first supply-discharge fluid tube 83 a andin the second supply-discharge fluid tube 83 b (the first fluid tube 83)in a direction indicated by an arrowed line A as shown in FIG. 2. Inaddition, when the auxiliary control valve 56C is at the second position91 b, the operation fluid flows in the first supply-discharge fluid tube83 a and in the second supply-discharge fluid tube 83 b (the first fluidtube 83) in a direction indicated by an arrowed line B.

In tins manner, when the operation member 99 is operated to switch theauxiliary control valve 56C to a predetermined position, the directionof flowing of the operation fluid can be changed in the firstsupply-discharge fluid tube 83 a and in the second supply-dischargefluid tube 83 b (the first fluid tube 83), and thereby controlling themovement of the hydraulic device connected to the first fluid tube 83.

The second fluid tube 84 is connected to the first supply-dischargefluid tube 83 a, that is, to the first fluid tube 83. The second fluidtube 84 is a fluid tube branching from an intermediate portion of thefirst supply-discharge fluid tube 83 a between the connection member 50and the auxiliary control valve 56C and extending toward the operationfluid tank 22.

In particular, one end of the second fluid tube 84 is connected to thefirst supply-discharge fluid tube 83 a, and the other end of the secondfluid tube 84 is connected to the operation fluid tank 22. A switchvalve 85 is disposed on an intermediate portion of the second fluid tube84 In addition, the oil cooler 81 is arranged on the second fluid tube84 between the switch valve 85 and the operation fluid tank 22.

To be detailed, the second fluid tube 84 includes a first drain fluidtube 84 a, a second drain fluid tube 84 b, and a third drain fluid tube84 c. The first drain fluid tube 84 a connects the firstsupply-discharge fluid tube 83 a to the switch valve 85. The seconddrain fluid tube 84 b connects the switch valve 85 to the oil cooler 81.The third drain fluid tube 84 c connects the oil cooler 81 to theoperation fluid tank 22. Meanwhile, a part of the second drain fluidtube 84 b is shared with a part of the sixth fluid tube 80.

In particular, a section T1 between the oil cooler 81 and a confluentportion 84 e is a fluid tube shared with the second drain fluid tube 84b and the sixth fluid tube 80, the confluent portion 84 e being aportion where an intermediate portion of the second drain fluid tube 84b and an intermediate portion of the sixth fluid tube 80 are confluenteach other.

In this manner, at least a part of the operation fluid returning fromthe hydraulic device can be supplied to the oil cooler 81 and theoperation fluid tank 22 through the second fluid tube 84 (the firstdrain fluid tube 84 a, the second drain fluid tube 84 b, and the thirddrain fluid tube 84 c).

In other words, the second fluid tube 84 is capable of supplying theoperation fluid to the oil cooler 81, bypassing the operation fluidreturning from the hydraulic device.

The switch valve 85 is a valve configured to change the aperture, and isconstituted of a two-position switch valve configured to be switched tothe first position 85 a and the second position 85 b. When the switchvalve 85 is at the first position 85 a, the aperture is substantiallyzero, and thus the switch valve 85 blocks the connection between thefirst drain fluid tube 84 a and the second drain fluid tube 84 b.

When the switch valve 85 is at the second position 85 b, the aperture isfully opened, and thus the switch valve 85 allows the communicationbetween the first drain fluid tube 84 a and the second drain fluid tube84 b.

The switch valve 85 is switched to the first position 85 a and to thesecond position 85 b by the pilot pressure applied to the auxiliarycontrol valve 56C. In particular, a fourth fluid tube 87 is arrangedbetween the first control fluid tube 86 a and the pressure-receivingportion 85 c to which the pilot pressure of the switch valve 85 isapplied. The fourth fluid tube 87 connects the pressure-receivingportion 85 c to the first control fluid tube 86 a.

In this manner, the pilot pressure of the pilot fluid of the firstcontrol fluid tube 86 a is applied to the pressure-receiving portion 85c through the fourth fluid tube 87.

Here, an aperture of the switch valve 85 is set based on a pressure ofthe pilot fluid applied to the fourth fluid tube 87 from the firstcontrol fluid tube 86 a that is the third fluid tube 86. That is, theaperture of the switch valve 85 is set based on the operation amountinputted to the controller 98.

When the aperture of the first electromagnetic valve 60A is small andthe pilot pressure of the pilot fluid of the first control fluid tube 86a is less than the setup value for example, the auxiliary control valve56C is at the neutral position that is the third position 91 c. In thatcase, the aperture of the switch valve 85 is less than the predeterminedvalue, and is at the first position 85 a.

That is, the switch valve 85 is also closed under that state where theauxiliary control valve 56C is at the third position 91 c and theoperation fluid is not supplied to the first control fluid tube 86 a.

On the other hand, when the aperture of the first electromagnetic valve60A is large and the pilot pressure of the pilot fluid of the firstcontrol fluid tube 86 a is the setup value or more, that is, when astroke of the spool of the auxiliary control valve 56C is apredetermined extent or more (two thirds or more), the auxiliary controlvalve 56C is switched to the first position 91 a. In that case, theaperture of the switch valve 85 is a predetermined value or more, andthus the switch valve 85 is switched to the second position 85 b.

In particular, the switch valve 85 is opened under the state where theauxiliary control valve 56C is switched to the first position 91 a andthe operation fluid flows in the first supply-discharge fluid tube 83 ain the direction indicated by the arrowed line A.

That is, the switch valve 85 is opened under the state where theoperation fluid of the hydraulic device has returned to the firstsupply-discharge fluid tube 83 a (when the first supply-discharge fluidtube 83 a serves as a fluid tube for supplying the returning fluid). Inthis manner, the operation fluid in the first control fluid tube 86 acan be supplied to the oil cooler 81 through the second fluid tube 84.

Thus, the operation fluid returning from the hydraulic device can beeasily cooled in synchronization with the movement of the auxiliarycontrol valve 56C (an operation of the operation valve 99).

In particular, a temperature of the operation fluid may be increased inaccordance with the revolving of the hydraulic motor in a case where theoperation member 99 is operated in one direction and a hydraulic motoris continuously driven in one direction, the hydraulic motor being oneof the hydraulic devices. In the embodiment, the operation fluidreturning from the hydraulic devices is supplied to the oil cooler 81through the second fluid tube 84, and thereby the temperature of theoperation fluid can be prevented from increasing.

For example, in the case where the auxiliary attachment is the hydraulicbreaker or the angle broom and the auxiliary attachment is continuouslydriven for a certain length of time by the hydraulic motor, theoperation fluid of the first supply-discharge fluid tube 83 a flows intothe oil cooler 81 through the second fluid tube 84. In this manner, thehydraulic breaker and the angle broom can be driven with the operationfluid cooled by the oil cooler 81.

Meanwhile, as shown in FIG. 2, a second check valve 78 may be disposedon the second fluid tube 84 between the switch valve 85 and theoperation fluid tank 22 in the work hydraulic system 30B. In particular,the second check valve 78 is arranged to the second drain fluid tube 84b. The second check valve 78 is a valve configured to block theoperation fluid flowing from the oil cooler 81 toward the switch valve85.

In this manner, the provision of the second check valve 78 supplies theoperation fluid returning from the hydraulic device to the oil cooler 81through the second fluid tube 84 even when various types of drain fluidtubes (for example, the sixth fluid tube 80 for supplying a drain fluidof the HST motor) other than the second fluid tube 84 are connected tothe oil cooler 81.

In addition, the second fluid tube 84 may be provided with a firstthrottle portion 79. In particular, the first throttle portion 79 isarranged on a side closer to the oil cooler 81 than the second checkvalve 78 in the second drain fluid tube 84 b of the second fluid tube84. The first throttle portion 79 is capable of suppressing (reducing) apressure of the operation fluid (the drain) flowing toward the oilcooler 81. Meanwhile, the first throttle portion 79 may be disposed onan intermediate portion of the first drain fluid tube 84 a of the secondfluid tube 84.

In addition, the first check valve 77 is disposed in the vicinity of atank port of the control valve unit. In this manner, the second fluidtube 84 easily supplies the operation fluid to the oil cooler 81 and theoperation fluid tank 22.

The hydraulic system 30B according to the first embodiment includes aload sensing system. The load sensing system is a system configured tocontrol an output rate of the second hydraulic pump P2 in accordancewith a work load. The load sensing system includes a first detectionfluid tube 70, a second detection fluid tube 71, a flow ratecompensation valve 72, and a swash plate control portion (a swash platecontroller) 73.

The first detection fluid tube 70 (referred to as a PLS fluid tube) is afluid tube connected to the control valves 56A, 56B, and 56C andconfigured to detect a load pressure generated when the control valves56A, 56B, and 56C are activated.

In addition, the first detection fluid tube 70 is also connected to theflow rate compensation valve 72, and transmits “a PUS signal pressure”to the flow rate compensation valve 72, the PLS signal pressure beingthe highest one of the load pressures of the control valves 56A, 56B,and 56C. The second detection fluid tube 71 (referred to as a PPS fluidtube) connects the flow rate compensation valve 72 to an output side ofthe second hydraulic, pump P2, and transmits “a PPS signal pressure” tothe flow rate compensation valve 72, the PPS signal pressure being anoutput pressure of the operation fluid outputted from the secondhydraulic pump P2.

The swash plate control portion 73 is a device having a piston, ahousing, and a rod. The piston is moved by the pressure of the operationfluid. The housing houses the piston. The rod is coupled to the piston.One end of the housing is connected to the flow rate compensation valve72, and the other end of the housing is connected to the output side ofthe second hydraulic pump P2. The rod (a moving portion) of the swashplate control portion 73 is connected to the swash plate of the secondhydraulic pump P2, and is stretched and shortened to change an angle ofthe swash plate.

The flow rate compensation valve 72 is a valve configured to control theswash plate control portion 73 on the basis of the PLS signal pressureand the PPS signal pressure. The flow rate compensation valve 72 appliesa pressure to one end side of the swash plate control portion 73 suchthat a differential pressure (a first differential pressure) between thePPS signal pressure and the PLS signal pressure can be a predeterminedpressure.

That is, the flow rate compensation valve 72 stretches and shortens therod disposed on the other end side of the swash plate control portion 73such that the differential pressure (the first differential pressure)between the PPS signal pressure and the PLS signal pressure can beconstant.

As described above, the load sensing system changes the angle of theswash plate such that the first differential pressure can be constant,and thereby the output rate of the second hydraulic pump P2 is adjustedon the basis of the load pressure. Meanwhile, the hydraulic systemaccording to the embodiment may be provided without the load sensingsystem.

Second Embodiment

A modified example of the hydraulic system according to a secondembodiment of the present invention will be explained below. FIG. 3shows a view illustrating the hydraulic system having two second fluidtubes 84 and arranging the switch valves 85 to each of the second fluidtubes 84. Explanation of configurations similar to the configurationsaccording to the first embodiment will be omitted.

The second fluid tube 84A is connected to the first supply-dischargefluid tube 83 a, and the second fluid tube 84B is connected to thesecond supply-discharge fluid tube 83 b. The switch valve 85A isdisposed on an intermediate portion of the second fluid tube 84A, andthe switch valve 85B is disposed on an intermediate portion of thesecond fluid tube 84B.

The second fluid tube 84A includes a first drain fluid tube 84 a, asecond drain fluid tube 84 b, and a third drain fluid tube 84 c. Thefirst drain fluid tube 84 a connects the first supply-discharge fluidtube 83 a to the switch valve 85A. The second drain fluid tube 84 bconnects the switch valve 85A to the oil cooler 81. The third drainfluid tube 84 c connects the oil cooler 81 to the operation fluid tank22.

The second fluid tube 84B includes a fourth drain fluid tube 84 d, thesecond drain fluid tube 84 b, and the third drain fluid tube 84 c. Thefourth drain fluid tube 84 d connects the second supply-discharge fluidtube 83 b to the switch valve 85A. The second drain fluid tube 84 bconnects the switch valve 85B to the oil cooler 81. The third drainfluid tube 84 c connects the oil cooler 81 to the operation fluid tank22. The second drain fluid tube 84 b and the third drain fluid tube 84 care shared with the second fluid tube 84A and the second fluid tube 84B.

In the embodiment, the hydraulic system includes two fluid tubes, afourth fluid tube 87 a and a fourth fluid tube 87 b. The fourth fluidtube 87 a is a fluid tube connecting the first control fluid tube 86 ato the pressure-receiving portion 85 c of the switch valve 85B. Thefourth fluid tube 87 b is a fluid tube connecting the second controlfluid tube 86 b to the pressure-receiving portion 85 c of the switchvalve 85B.

In this manner, the switch valve 85A is opened under the state where theoperation fluid flows in the direction indicated by the arrowed line Ain the first supply-discharge fluid tube 83 a. That is, the switch valve85A is opened under the state where the operation fluid of the hydraulicdevice has returned to the first supply-drain fluid tube 83 a, and thusthe operation fluid of the first control fluid tube 86 a can be suppliedtoward the oil cooler 81 through the second fluid tube 84A.

In addition, the switch valve 85B is opened under the state where theoperation fluid flows in the direction indicated by the arrowed line Bin the second supply-discharge fluid tube 83 b. That is, the switchvalve 85B is opened under the state where the operation fluid of thehydraulic device has returned to the second supply-drain fluid tube 83b, and thus the operation fluid of the second control fluid tube 86 bcan be supplied toward the oil cooler 81 through the second fluid tube84B.

In the embodiment mentioned above, the switch valve 85 is switched withused of the pilot pressures applied to the first control fluid tube 86 aand the second control fluid tube 86 b. However, instead of that, theswitch valve 85 may be switched by the pilot fluid outputted from thefirst hydraulic pump P1 as shown in FIG. 4A.

In that case, another valve (a switch valve or the like) 75 forswitching the switch valve 85 may be disposed between the firsthydraulic pump P1 and the pressure-receiving portion 85 c of the switchvalve 85, the other valve being configured to be opened and closed. Inthat case, the pilot pressure is applied to the pressure-receivingportion 85 c of the switch valve 85 by opening the valve (a secondoperation valve) 75 for example, and thereby the switch valve 85 isswitched to the second position 85 b.

In addition, the pilot pressure is not applied to the pressure-receivingportion 85 c of the switch valve 85 by closing the valve 75, and therebythe switch valve 85 is switched to the first position 85 a. Meanwhile,the valve 75 illustrated in FIG. 4A may be a proportional valve insteadof the switch valve mentioned above.

Moreover, as shown in FIG. 4B, the switch valve 85 may be a valveconfigured to be magnetized to change the aperture. In thatconfiguration, the solenoid 85 d of the switch valve 85 is connected tothe controller 98. Then, the controller 98 outputs a control signal (forexample, an electric current) from the controller 98 to the switch valve85, and thereby the solenoid 85 d is magnetized. In this manner, theswitch valve 85 is switched to the second position 85 b.

In addition, the output of the control signal from the controller 98 tothe switch valve 85 is stopped, the solenoid is demagnetized, andthereby the switch valve 85 is switched to the first position 85 a. Thecontrol signal from the controller 98 to the switch valve 85 may beoutputted by a switch or the like connected to the controller 98 forexample. When a cooling mode or a motor attachment mode is set by theoperation of the switch, the control signal is outputted to the switchvalve 85, and when the cooling mode or the motor attachment mode isreleased, the output of the control signal to the switch valve 85 isstopped.

The cooling mode is a mode for cooling the operation fluid in the oilcooler 81. The motor attachment mode is a mode to be set when theauxiliary attachment to be driven by the motor is attached.

The switch valve 85 may be switched in the case where the temperature ofthe operation fluid is high. For example, the first fluid tube 84 may beprovided with a measurement device configured to detect the temperatureof the operation fluid, and thus the controller 98 may be in the coolingmode when the temperature of the operation fluid measured by themeasurement device is a predetermined valve (a threshold value) or more.

The cooling mode of the controller 98 is released when the temperatureof the operation fluid measured by the measurement device is less thanthe threshold value. The controller 98 may open the switch valve 85 whenthe fluid returning from the auxiliary actuator increases and theoperation fluid flowing to the auxiliary actuator increases.

That is, the switch valve 85 is opened when the stroke of the spool ofthe auxiliary control valve 56C exceeds a predetermined threshold value,when the spool of the auxiliary control valve 56C stops moving.

In the embodiment mentioned above, the oil cooler 81 cools the fluidreturning from the travel motors (the first travel motor 31L and thesecond travel motor 31R) and the fluid returning from the hydraulicdevice. However, instead of that, the hydraulic system is configured asshown in FIG. 5, and thereby cool the fluid returning from the controlvalve 56 may be cooled.

As shown in FIG. 5, a first oil cooler (a first fluid cooler) 81A isconnected to an end portion of the second drain fluid tube 84 b and toan end portion of the sixth fluid tube 80.

The third drain fluid tube 84 c connects the first oil cooler 81A to theoperation fluid tank 22. A second oil cooler (a second fluid cooler) 81Bis connected to the fifth fluid tube 82 between the first check valve 77and the operation fluid tank 22.

In this manner, the operation fluid returning from the control valves(the boom control valve 56A, the bucket control valve 561, and the like)(a returning operation fluid) can be cooled by the second oil cooler81B. The operation fluid returning from the travel motor and thehydraulic devices (a returning operation fluid) can be cooled by thefirst oil cooler 81A.

In particular, the returning operation fluids flow into the first oilcooler 81A when the travel motor and the like are operated. In addition,the returning operation fluids flow into the first oil cooler 81A whenthe auxiliary control valve 56 c (the hydraulic device) are operated.

In FIG. 5, the second fluid tube 84 and the fifth fluid tube 82 areseparately connected to the operation fluid tank 22. However, thehydraulic system includes the first check valve 77 configured to supplythe operation fluid having a predetermined pressure or more in only onedirection, and thus the operation fluid returning from the hydraulicdevice is easily supplied to the second fluid tube 84.

Third Embodiment

A modified example of the hydraulic system according to a thirdembodiment of the present invention will be explained below. Explanationof configurations similar to the configurations according to the firstembodiment and the second embodiment will be omitted. The hydraulicsystem according to the third embodiment does not include the oil cooler81 mentioned above.

As shown in FIG. 6, the plurality of control valve 56 (the boom controlvalve 56A, the bucket control valve 56B, and the auxiliary control valve56C) are disposed on a control valve (a control valve unit) 65.

A first operation valve 101 is disposed on an intermediate portion ofthe second fluid tube 84. In particular, the second fluid tube 84includes a fifth drain fluid tube 84 e and a sixth drain fluid tube 84f. The fifth drain fluid tube 84 e connects the first supply-dischargefluid tube 83 a to the first operation valve 101. The sixth drain fluidtube 84 f connects the first operation valve 101 to the operation fluidtank 22. Meanwhile, a part of the sixth drain fluid tube 84 f is sharedwith a part of the sixth fluid tube 80.

The first operation valve 101 is a valve configured to change theaperture of the first operation valve 101, and is constituted of atwo-position switch valve configured to be switched to a first position101 a and to a second position 101 b. When the first operation valve 101is at the first position 101 a, the aperture of the first operationvalve 101 is substantially zero, and thereby a connection between thefifth drain fluid tube 84 e and the sixth drain fluid tube 84 f isclosed. When the first operation valve 101 is at the second position 101b, the aperture of the first operation valve 101 is fully opened, andthereby the connection between the fifth drain fluid tube 84 e and thesixth drain fluid tube 84 f is opened.

The first operation valve 101 includes a pressure-receiving portion 101c. The pressure-receiving portion 101 c is configured to receive thepressure of the operation fluid, and a seventh fluid tube 105 isconnected to the pressure-receiving portion 101 c. End portion of theseventh fluid tube 105 is connected to the first control fluid tube 86a, the end portion (the other end portion) being opposite to the endportion (one end portion) connected to the pressure-receiving portion101 c.

As described above, when one end of the seventh fluid tube 105 isconnected to the pressure-receiving portion 101 c and the other end ofthe seventh fluid tube 105 is connected to the first control fluid tube86 a, the seventh fluid tube 105 has the same configuration as theconfiguration of the fourth fluid tube 87.

A second operation valve 102 is disposed on an intermediate portion ofthe seventh fluid tube 105. The second operation valve 102 is a valvefor switching the first operation valve 101, and is configured to changethe aperture of the second operation valve 102. The second operationvalve 102 is constituted of an electromagnetic valve or a manual valve,the manual valve being configured to manually change the aperture. Inthe case where the second operation valve 102 is constituted of themanual valve, the pilot pressure to be applied to the pressure-receivingportion 101 c of the first operation valve 101 can be set by manuallyopening or closing the second operation valve 102 fully.

That is, the hydraulic system according to the third embodiment includesthe second fluid tube 84, the first operation valve 101, the secondoperation valve 102, and the seventh fluid tube 105, and thus is capableof changing a flowing path of the operation fluid (the returningoperation fluid) returning from the auxiliary hydraulic actuator.

For example, the auxiliary attachment is a breaker, the second operationvalve 102 is fully opened. In this manner, the pilot pressure of thepilot fluid of the first control fluid tube 86 a is applied to thepressure-receiving portion 101 c, and thereby the first operation valve101 is switched to the second position, 101 b.

The operation fluid (the returning operation fluid) returns to the firstsupply-discharge fluid tube 83 a from the hydraulic device configured tooperate the breaker. The returning operation fluid passes through thesecond fluid tube 84 and the first operation valve 101, and then issupplied to the operation fluid tank 22.

Thus, in the case where the auxiliary attachment is the breaker or thelike and the operation fluid returning from the breaker tends to bepulsated, the returning operation fluid can be supplied to the operationfluid tank 22 without passing through the control valve (the controlvalve unit) 65 having the auxiliary control valve 56C when the secondoperation valve 102 is fully opened.

In addition, when the second operation valve 102 is fully closed, thepilot pressure of the pilot fluid of the first control fluid tube 86 ais not applied to the pressure-receiving portion 101 c. In this manner,the first operation valve 101 is switched to the first position 101 a.In the case where the first operation valve 101 is switched to the firstposition 101 a, the returning operation fluid of the firstsupply-discharge fluid tube 83 a passes through the auxiliary controlvalve 56C and then flows into the operation fluid tank 22.

The hydraulic system according to the third embodiment may include theoil cooler according to the first embodiment or the second embodiment.The opening and closing of the first operation valve 101 may becontrolled by the controller 98 in the same manner according to theembodiments mentioned above.

In addition, the conditions to open and close the first operation valve101 with the controller 98 may be the same as the conditions to theswitch valve 85. That is, the control of the controller 98 according tothe second embodiment may be applied to the first operation valve 101instead of the switch valve 85.

Fourth Embodiment

A modified example of the hydraulic system. according to a fourthembodiment of the present invention will be explained below. Explanationof configurations similar to the configurations according to the thirdembodiment will be omitted. The hydraulic system according to the fourthembodiment does not include the oil cooler 81 and the load sensingsystem mentioned above. The hydraulic system according to the fourthembodiment has a configuration capable of increasing the operationfluid.

As shown in FIG. 7A, the hydraulic system 30B includes a third hydraulicpump P3, the third operation valve 120, a fourth operation valve 121,and an eighth fluid tube 123.

The third hydraulic pump P3 is a pump configured to be driven by amotive power of the prime mover 32, and is a pump arranged on a positiondifferent from the first hydraulic pump P1 and the second hydraulic pump(the work hydraulic pump) P2. The second hydraulic pump and the thirdhydraulic pump P3 are pumps configured to be driven by the motive powerof the prime mover 32, and are constituted of the constant displacementgear pumps.

That is, each of the first hydraulic pump P1, the second hydraulic pumpP2, and the third hydraulic pump P3 is constituted of the constantdisplacement gear pump in the fourth embodiment. In particular, thethird hydraulic pump P3 is a pump for additionally increasing theoperation fluid, that is, a hydraulic pump for increment.

The eighth fluid tube 123 is a fluid tube branching from the first fluidtube 83 and being connected to the third hydraulic pump P3. Inparticular, one end of the eighth fluid tube 123 is connected to tofirst supply-discharge fluid tube 83 a, and the other end of the eighthfluid tube 123 is connected to an output side of the third hydraulicpump P3.

The third operation valve (a high flow valve) 120 is a valve disposed onan intermediate portion of the eighth fluid tube 123, and configured tochange the aperture of the third operational valve 120. The thirdoperation valve 120 is constituted of a two-position switch valveconfigured to be operated by the pilot pressure. The third operationvalve 120 is configured to be switched to two switching position (thefirst position 120 a and the second position 120 b) by the pilotpressure.

When the third operation valve 120 is at the first position 20 a, theaperture of the third operation valve 120 is substantially zero, andthereby the flow rate of the operation fluid flowing in the eighth fluidtube 123 becomes zero. In addition, when the third operation valve 120is at the second position 120 b, the aperture of the third operationvalve 120 is fully opened, and thereby the now rate of the operationfluid flowing in the eighth fluid tube 123 becomes a predetermined flowrate from zero.

In other words, the third operation valve 120 closes the eighth fluidtube 123 when the third operation valve 120 is at the first position 120a, and the third operation valve 120 opens the eighth fluid tube 123when the third operation valve 120 is at the second position 120 b.

Thus, when the third operation valve 120 is set to the second position120 b, the operation fluid outputted from the third hydraulic pump P3can be supplied to the eighth fluid tube 123. The operation fluidflowing to the eighth fluid tube 123 is confluent with the operationfluid flowing to the first fluid tube 83. As the result, the operationfluid to be supplied to the auxiliary actuator can be additionallyincreased.

The third operation valve 120 is switched by the fourth operation valve(the switch valve) 121. The fourth operation valve 121 is constituted ofan electromagnetic two-position switch valve. The fourth operation valve121 is configured to be switched to the first position 121 a and to thesecond position 121 b. The fourth operation valve 121 is connected tothe third operation valve 120 by a ninth fluid tube 125.

In particular, the third operation valve 120 includes apressure-receiving portion 120 c. The pressure-receiving portion 120 cis configured to receive the pressure of the pilot fluid. Thepressure-receiving portion 120 c of the third operation valve 120 isconnected to the fourth operation valve 121 by the ninth fluid tube 125.

When the fourth operation valve 121 is at the first position 121 a, thepilot pressure is not applied to the pressure-receiving portion 120 c ofthe third operation valve 120, and the third operation valve 120 isswitched to the first position 120 a. When the fourth operation valve121 is at the second position 121 b, the pilot pressure is applied tothe pressure-receiving portion 120 c of the third operation valve 120,and the third operation valve 120 is switched to the second position 120b.

The fourth operation valve 121 is switched to the first position 121 aand to the second position 121 b by the controller 98. When the switchis operated to the maximum extent, the controller 98 continuouslymagnetizes the solenoid of the fourth operation valve 121. In. thismanner, the fourth operation valve 121 is switched to the secondposition 121 b.

When the switch is not operated to the maximum extent, the controller 98demagnetizes the solenoid of the fourth operation valve 121. In thismanner, the fourth operation valve 121 is switched to the first position121 a.

End portion of the seventh fluid tube 105 (a side connected to thepressure-receiving portion 101 c) is connected to an intermediateportion of the ninth fluid tube 125, the end portion (the other endportion) being opposite to the end portion (one end portion) connectedto the pressure-receiving portion 101 c. In addition, the secondoperation valve 102 is connected to an intermediate portion of theseventh fluid tube 105.

Meanwhile, a section 105 b between the second operation valve 102 andthe connection portion 105 a of the ninth fluid tube 125 is connected tothe first control fluid tube 86 a of the third fluid tube 86 in theseventh fluid tube 105. A check valve 128 is disposed on the section 105b of the seventh fluid tube 105, the check valve 128 being configured toallow the operation fluid to flow from the side of the ninth fluid tube125 to the side of the second operation valve 102 and to block theoperation fluid from flowing from the side of the second operation valve102 to the side of the ninth fluid tube 125.

Fifth Embodiment

A modified example of the hydraulic system according to a fifthembodiment of the present invention will be explained below. Thehydraulic system according to the fifth embodiment includes the oilcooler 81 and the load sensing system mentioned above. However, the loadsensing system is not necessarily required in the hydraulic systemaccording to the fifth embodiment. Explanation of configurations similarto the configurations according to the embodiments mentioned above willbe omitted.

As shown in FIG. 8A, a fifth operation valve 130 is disposed on thefirst fluid tube 83. In particular, the fifth operation valve 130 isdisposed on an intermediate portion of the second supply-discharge fluidtube 83 b. In addition, the fifth operation valve 130 is provided withthe fifth fluid tube 82. The fifth operation valve 130 is constituted ofa two-position switch valve configured to be switched to a firstposition 130 a and to a second position 130 b.

When the fifth operation valve 130 is at the first position 130 a, thesecond supply-discharge fluid tube 83 b is communicated with the fifthfluid tube 82. When the fifth operation valve 130 is at the secondposition 130 b, the second supply-discharge fluid tube 83 b is opened(communicated).

The fifth operation valve 130 includes a pressure-receiving portion 130c. The pressure-receiving portion 130 c is configured to receive thepressure of the pilot fluid, and is connected to a tenth fluid tube 131.End portion of the tenth fluid tube 131 (a side connected to thepressure-receiving portion 130 c) is connected to the second controlfluid tube 86 b, the end portion (the other end portion) being oppositeto the end portion (one end portion) connected to the pressure-receivingportion 130 c.

A sixth operation valve 132 is disposed on an intermediate portion ofthe tenth fluid tube 131. The sixth operation valve 132 is a valveconfigured to switch the fifth operation valve 130. The sixth operationvalve 132 may be constituted of an electromagnetic valve or a manualvalve. In the embodiment, the sixth operation valve 132 is constitutedof an electromagnetic two-position switch valve configured to beswitched to a first position 132 a and to the second position 132 b.

When the sixth operation valve 132 is at the first position 132 a, theoperation fluid is allowed to flow from the side of the secondsupply-discharge fluid tube 83 b to the pressure-receiving portion 130 cof the fifth operation valve 130 in the tenth fluid tube 131. When thesixth operation valve 132 is at the second position 132b, the operationfluid is blocked from flowing to the operation fluid tank 22 in thetenth fluid tube 131.

Thus, when the sixth operation valve 132 is at the first position 132 a,a predetermined pilot pressure is applied to the pressure-receivingportion 130 c, and thus the fifth operation valve 130 is switched to thesecond position 130 b. When the sixth operation valve 132 is at thesecond position 132 b, a pilot pressure is not applied to thepressure-receiving portion 130 c, and thus the fifth operation valve 130is switched to the first position 130 a.

Meanwhile, the sixth operation valve 132 is switched by the switch orthe like connected to the controller 98.

FIG. 8B shows a modified example of the fifth operation valve 130according to the fifth embodiment. The fifth operation valve 130 opensthe second supply-discharge fluid tube 83 b when the fifth operationvalve 130 is at the first position 130 a. In addition, the fifthoperation valve 130 communicates the second supply-discharge fluid tube83 b with the fifth fluid tube 82 when the fifth operation valve 130 isat the second position 130 b. In the hydraulic system shown in FIG. 8B,the configurations other than the fifth operation valve 130 is similarto the configurations shown in FIG. 8A.

Sixth Embodiment

A modified example of the hydraulic system according to a sixthembodiment of the present invention will be explained below. Thehydraulic system according to the sixth embodiment includes the oilcooler 81 and the load sensing system mentioned above. However, the loadsensing system is not necessarily required in the hydraulic systemaccording to the fifth embodiment. Explanation of configurations similarto the configurations according to the embodiments mentioned above willbe omitted.

As shown in FIG. 9, the second fluid tube 84 includes the first drainfluid tube 84 a and a seventh drain fluid tube 84 g. The first drainfluid tube 84 a connects the first supply-discharge fluid tube 83 a. Theseventh drain fluid tube 84 r connects the switch valve 85 to theoperation fluid tank 22. The fifth fluid tube 82 is connected to anintermediate portion of the seventh drain fluid tube 84 g. A third cheekvalve 135 is disposed on an intermediate portion of the seventh drainfluid tube 84 g.

The third check valve 135 blocks the operation fluid having apredetermined pressure or more from flowing from a side of the operationfluid tank 22 to the second fluid tube 84 and allows the operation fluidto flow from a side of the second fluid tube 84 to the operation fluidtank 22.

The fifth fluid tube 82 is connected to the operation fluid tank 22separately from the seventh drain fluid tube 84 g. That is, the fifthfluid tube 82 branches at the intermediate portion and is connected tothe seventh drain fluid tube 84 g and the operation fluid tank 22. Theoil cooler 81 is connected to the fifth fluid tube 82.

In this manner, the provision of the switch valve 85 does not require toreturn the operation fluid returning from the connection member 50 (theauxiliary actuator) to the operation fluid tank 22 through the auxiliarycontrol valve 56C (the spool), and thereby reducing a pressure loss ofthe operation fluid.

In addition, the returning operation fluid can pass through the oilcooler 81 in both of the cases where the returning operation fluid isbypassed through the switch valve 85 and where the returning operationfluid is not bypassed through the switch valve 85, thereby cooling theoperation fluid efficiently.

Meanwhile, the hydraulic system shown in FIG. 9 may have a configurationwhere the second operation valve 102 is disposed on the fourth fluidtube 87 and may have a configuration where the second operation valve102 is not provided as shown in FIG. 9.

In the embodiment mentioned above, the flowing path of the operationfluid (the returning operation fluid) returning from the auxiliaryhydraulic actuator is changed. However, the configuration may be appliedto the circuits such as the travel device, the device, and the like. Thetravel device is explained as an example.

FIG. 10 is a schematic view illustrating a hydraulic device of the turndevice. The hydraulic circuit for the turn device is not limited to thecircuit shown in FIG. 10. In addition, he hydraulic circuit of the turndevice may be a closed circuit and may be an open circuit. The hydrauliccircuit of the turn device can be applied to a work machine such as abackhoe and the like.

As shown in FIG. 10, the turn device is one of the hydraulic devicesoperated by the operation fluid, and includes a turn motor 150 and aswitch valve 151. The switch valve 151 is configured to switch a tumdirection of the turn motor 150 between a normal turn and a reverseturn. The turn motor 10 and the switch valve 151 are connected to afluid tube 157. The switch valve 151 is connected to a turn controlvalve 152 by the first fluid tube 83. The turn control valve 152 controlthe turning.

The second fluid tube 84 is connected to the first fluid tube 83. Thefirst operation valve 101 is connected to the second fluid tube 84. Acheck valve 155 is disposed on a downstream side of the second fluidtube 84. The seventh fluid tube 105 connects the pressure-receivingportion 101 c of the first operation valve 101 to the pressure-receivingportions 151 a and 151 b of the switch valve 151. A throttle portion 156is connected to the seventh fluid tube 105.

The hydraulic system according to the embodiment is configured toadequately cool the operation fluid even when the hydraulic device areoperated by the operation fluid. In addition, the hydraulic systemaccording to the embodiment is configured to change the path of theoperation fluid (tile returning operation fluid) returning from thehydraulic actuator.

In the above description, the embodiment of the present invention hasbeen explained. However, all the features of the embodiments disclosedin this application should be considered just as examples, and theembodiments do not restrict the present invention accordingly. A scopeof the present invention is shown not in the above-described embodimentsbut in claims, and is intended to include all modifications within andequivalent to a scope of the claims.

The first check valve 77, the second check valve 78, the first throttleportion 79, and the like may be arranged in the control valve unit thathas the plurality of control valves 56. And, the switch valve 101, thefirst cheek valve 77, the second check valve 78, the first throttleportion 79, and the like may be configured separately from the controlvalve unit.

In addition, a whole of or a part of the second fluid tube 84 may beincluded in the control valve unit. The first electromagnetic valve 60Aand the second electromagnetic valve 60B may be arranged in the controlvalve unit. The first electromagnetic valve 60A and the secondelectromagnetic valve 60B may be configured separately from the controlvalve unit.

In addition, the second hydraulic pump P2 may be a constant displacementpump, and may be other types of pumps. The hydraulic system may includetwo constant displacement pumps to increase the operation fluid to besupplied to the hydraulic devices.

That is, the hydraulic systems according to the embodiments can beapplied to the hydraulic system of a high flow specification thatsupplies the operation fluid by activating two constant displacementpumps or one of the constant displacement pumps depending on theauxiliary actuator.

In the embodiments mentioned above, the switch valve 85 is constitutedof a two-position switch valve. However, instead of that, the switchvalve 85 may be constituted of a pilot check valve that is configured tobe operated by the pilot pressure. For example, a fluid tube isconnected to a pressure-receiving portion of the pilot check valve, apressure of the operation fluid (the pilot fluid) is applied to thepressure-receiving portion, band thereby the pilot check valve isopened. In addition, the pilot valve is closed by preventing thepressure of the operation fluid from being applied to thepressure-receiving portion of the pilot check valve,

In addition, regarding the above-mentioned valve having the spool, thespool may be moved by a hydraulic operation (a hydraulic operation bythe pilot valve, a hydraulic operation by the proportional valve), by anelectric operation (an electric operation by magnetizing the solenoid),or by other methods (other operations).

In the embodiments mentioned above, the operation fluid drained to theoperation fluid tank. However, the operation fluid may be drained toother devices. That is, a fluid tube (a fluid path) for draining(discharging) the operation fluid may be connected to a device otherthan the operation fluid tank. For example, the fluid tube may beconnected to a suction portion of the hydraulic pump (a portion to suckthe operation fluid) and may be connected to other portions.

What is claimed is:
 1. A hydraulic system for a work machine comprising:a tank to store an operation fluid; a hydraulic device to be operated bythe operation fluid; a control valve to control the hydraulic device; afirst fluid tube connecting the hydraulic device and the control valvefor fluid communication of the operation fluid therebetween; a secondfluid tube branching from the first fluid tube to allow the operationfluid therein to be drained to the tank; a switch valve provided to thesecond fluid tube to control a flow rate and/or a pressure of theoperation fluid; an oil cooler provided to the second fluid tube betweenthe switch valve and the tank; a fifth fluid tube connecting the controlvalve and the tank; and a check valve provided to the fifth fluid tubeto allow the operation fluid to be drained from the control valve to thetank through the fifth fluid tube only if the drained operation fluidhas a pressure equal to or greater than a predetermined pressure.
 2. Thehydraulic system according to claim 1, wherein the fifth fluid tube isconnected to the second fluid tube for fluid communication of theoperation fluid therebetween, and wherein the check valve allows thefluid communication of the operation fluid from the fifth fluid tube tothe second fluid tube and prevent the fluid communication of theoperation fluid from the second fluid tube to the fifth fluid tube. 3.The hydraulic system according to claim 1, wherein the hydraulic deviceincludes a hydraulic actuator to actuate an auxiliary attachment, andwherein the control valve includes an auxiliary control valve to controla flow rate and/or a pressure of the operation fluid supplied to thehydraulic actuator.
 4. The hydraulic system according to claim 1,wherein the first fluid tube includes a first supply-discharge fluidtube and a second supply-discharge fluid tube both of which connects thehydraulic device and the control valve for fluid communication of theoperation fluid therebetween, and wherein one of the firstsupply-discharge fluid tube and the second supply-discharge fluid tubeis used for fluid communication of the operation fluid from thehydraulic device to the control valve, and the other one of the firstsupply-discharge fluid tube and the second supply-discharge fluid tubeis used for fluid communication of the operation fluid from the controlvalve to the hydraulic device.
 5. The hydraulic system according toclaim 4, wherein the second fluid tube branches from one of the firstsupply-discharge fluid tube and the second supply-discharge fluid tubeto allow the operation fluid therein to be drained to the tank.